1. Field of the Invention
The present invention relates to a method of phosphating metal surfaces, and more specifically, it relates to a treatment method for forming a phosphate coating on metal surfaces of a combination of iron steel and/or galvanized steel and an aluminum alloy.
2. Description of the Background Art
Conversion treatment of forming a phosphate coating, which is basically composed of zinc phosphate, on a metal surface for improving finishing in coating and a rust preventing function is generally well known in the art.
While steel and galvanized steel are generally employed as metal materials which are subjected to conversion treatment, a material prepared by combining steel and/or galvanized steel with an aluminum alloy is recently employed in order to attain lightweightness in an automobile body or the like. Thus, awaited is a treatment method of simultaneously forming a phosphate coating which is basically composed of zinc phosphate on such a composite metal material, similarly to the conventional method.
As to conversion treatment for such a metal material which is prepared by combining iron steel or galvanized steel with an aluminum alloy, each of Japanese Patent Laying-Open Nos. 63-15789 (1988) and 64-68481 (1989) discloses a treatment method employing a phosphate treatment solution containing a fluoride. In the technique disclosed in either gazette, however, it is impossible to form a homogeneous and excellent phosphate coating in any or all metal surfaces of the steel, the galvanized steel and the aluminum alloy, and no sufficient performance can be attained as a cation coating substrate.
On the other hand, Japanese Patent Laying-Open No. 3-267378 (1991) discloses a treatment method of forming a homogeneous phosphate coating on a structure which consists of steel, galvanized steel and an aluminum alloy while defining relations between the treatment temperature and amounts of fluorine ions and zinc ions. According to the method which is disclosed in this gazette, it is possible to form a homogeneous phosphate coating on metal materials of a combination of steel, galvanized steel and an aluminum alloy, thereby obtaining a conversion coating having excellent corrosion resistance.
In recent years, however, awaited is a treatment method capable of forming a conversion coating having superior corrosion resistance, which exhibits excellent corrosion resistance also in a composite corrosion cycle test, being a severer corrosion resistance test.